Stimulated Raman Scattering In High Power Fiber Lasers

The stimulated Raman scattering(SRS) effect is the dominant nonlinear process in the continuous wave(CW) high power fiber lasers(HPFLs), which sets the upper limit to the power scaling capabilities of the fiber lasers. The SRS effect in HPFLs is the main research object of this thesis, targetting to improve the output power of the fiber laser. Three hierarchies of research work are carried out in this thesis. Firstly, we study the evolution, limiting factors and suppression method of the SRS effect in ytterbium(Yb) doped fiber lasers. Secondly, the forward SRS compatible fiber laser scheme is proposed, in which the backward SRS is suppressed by injecting a forward Raman seed. Both the Raman gain and Yb gain are utilized so as to enhance the output power of Yb doped fiber lasers(YDFLs). Thirdly, we research on Raman fiber amplifier to obtain high power, high efficiency and high brightness laser output by only using Raman gain in the fiber. The main work of the thesis is as follows:1. The SRS effect in CW YDFLs is theoretically and experimentally studied. Firstly, the Raman gain coefficient of the passive fiber is measured using the small signal gain method, while that of the active fiber is obtained by employing the demodulation method. It provides the basis for the type choosing of fibers in lasers and for the numerical simulation in the thesis. Secondly, based on the rate equations of the YDFLs and coupling equations of the SRS effect, the numerical models are established to study the SRS effect in the high power fiber oscillator and amplifier. The numerical and experimental results show that forward and backward scattering light will be excited when the power of signal in the fiber laser reaches SRS threshold, and the bidirectional Stokes power are in the same order of magnitude. In a CW master-oscillator power amplifiers(MOPA) system, the backward Raman scattering light excited in the amplifier will be amplified while passing through the oscillator, meanwhile the output power of the fiber oscillator will be decreased; what’s worse the laser system will be destroyed. Therefore, the backward Raman scattering becomes one of the main limiting factors that restrict the power scaling of CW fiber lasers.2. The influence of self-pulsation on the performance of fiber amplifier is studied especially the SRS effect. Firstly, the characteristics, physical mechanism and suppression methods ofself-pulsation are presented. Secondly, an improved model of fiber amplifier considering the temporal characteristics and SRS effect is established, which is composed of a set of instantaneous-state propagation-rate equations. The numerical results show that the self-pulsation effect induces power transfer from signal laser to forward Stokes light, and has little influence on the backward Stokes light. Thirdly, three watt-level seed sources with different self-pulsation contents are amplified by the same fiber amplifier respectively. The influence of self-pulsation content on the threshold of SRS effect and power scaling is studied. The experimental results show that the increasement of self-pulsation content will excite more Stokes light, and the power of the forward Stokes light is much larger than that of the backward Stokes. Lastly, the influence of self-pulsation in a three kilowatts fiber laser on the SRS effect and power scaling is studied. The results show that the self-pulsation effect considerably decreases the threshold of forward SRS, and sets the upper limit to the power scaling capabilities of fiber lasers.3. The forward SRS compatible fiber laser system is proposed and the restricted laser power is promoted by suppressing the backward SRS. Firstly, the concept of forward SRS compatible fiber laser scheme is given. The forward Stokes light excited by SRS effect is acceptable, while the backward Stokes light is the limiting factor of the power scaling of HPFLs. Secondly, a suppression method of the backward Stokes light is proposed. The experimental results show that the backward light power in the MOPA structure HPFLs can be reduced by employing a forward Raman seed. Lastly, a high power forward SRS compatible and tandem pumped fiber laser is designed. The influence of self-pulsation and forward Raman seed on the fiber laser power scaling capability is researched numerically. The simulative results show that the output power can be enhanced by employing forward Raman seed.4. The theoretical and experimental research on the key technologies of Raman fiber laser is presented in an attempt to achieve a high efficient and high beam quality Raman fiber laser system. Firstly, the laser brightness enhancement in the core pumping Raman fiber amplifier is studied. The results show that SRS effect in the graded-index fibers has the beam cleanup effect, which does not occur in step-index fibers. Secondly, the output characteristics of the clad pumping Raman fiber amplifier are studied. A special refractive index distribution double-clad Raman fiber is designed for the cladding pumped Raman fiber amplifier, in which the signal laser beam quality is improved by SRS effect. Due to the large pump light loss coefficient in the inner cladding of Raman fiber, the efficiency of the Raman fiber amplifier is low. Lastly, the key technologies of high efficient and high beam quality RFL are studied. The results show that a Raman filter fiber with low loss to the pump and signal light is needed to achieve the Raman fiber laser system with high power, high efficient and high beam quality.